1Documentation for /proc/sys/kernel/* kernel version 2.2.10
2 (c) 1998, 1999, Rik van Riel <riel@nl.linux.org>
3 (c) 2009, Shen Feng<shen@cn.fujitsu.com>
4 5For general info and legal blurb, please look in README.
6 7==============================================================
8 9This file contains documentation for the sysctl files in
10/proc/sys/kernel/ and is valid for Linux kernel version 2.2.
11 12The files in this directory can be used to tune and monitor
13miscellaneous and general things in the operation of the Linux
14kernel. Since some of the files _can_ be used to screw up your
15system, it is advisable to read both documentation and source
16before actually making adjustments.
17 18Currently, these files might (depending on your configuration)
19show up in /proc/sys/kernel:
20 21- acct
22- acpi_video_flags
23- auto_msgmni
24- bootloader_type [ X86 only ]
25- bootloader_version [ X86 only ]
26- callhome [ S390 only ]
27- cap_last_cap
28- core_pattern
29- core_pipe_limit
30- core_uses_pid
31- ctrl-alt-del
32- dmesg_restrict
33- domainname
34- hostname
35- hotplug
36- kptr_restrict
37- kstack_depth_to_print [ X86 only ]
38- l2cr [ PPC only ]
39- modprobe ==> Documentation/debugging-modules.txt
40- modules_disabled
41- msgmax
42- msgmnb
43- msgmni
44- nmi_watchdog
45- osrelease
46- ostype
47- overflowgid
48- overflowuid
49- panic
50- panic_on_oops
51- panic_on_unrecovered_nmi
52- panic_on_stackoverflow
53- pid_max
54- powersave-nap [ PPC only ]
55- printk
56- printk_delay
57- printk_ratelimit
58- printk_ratelimit_burst
59- randomize_va_space
60- real-root-dev ==> Documentation/initrd.txt
61- reboot-cmd [ SPARC only ]
62- rtsig-max
63- rtsig-nr
64- sem
65- sg-big-buff [ generic SCSI device (sg) ]
66- shm_rmid_forced
67- shmall
68- shmmax [ sysv ipc ]
69- shmmni
70- softlockup_thresh
71- stop-a [ SPARC only ]
72- sysrq ==> Documentation/sysrq.txt
73- tainted
74- threads-max
75- unknown_nmi_panic
76- version
77 78==============================================================
79 80acct:
81 82highwater lowwater frequency
83 84If BSD-style process accounting is enabled these values control
85its behaviour. If free space on filesystem where the log lives
86goes below <lowwater>% accounting suspends. If free space gets
87above <highwater>% accounting resumes. <Frequency> determines
88how often do we check the amount of free space (value is in
89seconds). Default:
904 2 30
91That is, suspend accounting if there left <= 2% free; resume it
92if we got >=4%; consider information about amount of free space
93valid for 30 seconds.
94 95==============================================================
96 97acpi_video_flags:
98 99flags
100 101See Doc*/kernel/power/video.txt, it allows mode of video boot to be
102set during run time.
103 104==============================================================
105 106auto_msgmni:
107 108Enables/Disables automatic recomputing of msgmni upon memory add/remove
109or upon ipc namespace creation/removal (see the msgmni description
110above). Echoing "1" into this file enables msgmni automatic recomputing.
111Echoing "0" turns it off. auto_msgmni default value is 1.
112 113 114==============================================================
115 116bootloader_type:
117 118x86 bootloader identification
119 120This gives the bootloader type number as indicated by the bootloader,
121shifted left by 4, and OR'd with the low four bits of the bootloader
122version. The reason for this encoding is that this used to match the
123type_of_loader field in the kernel header; the encoding is kept for
124backwards compatibility. That is, if the full bootloader type number
125is 0x15 and the full version number is 0x234, this file will contain
126the value 340 = 0x154.
127 128See the type_of_loader and ext_loader_type fields in
129Documentation/x86/boot.txt for additional information.
130 131==============================================================
132 133bootloader_version:
134 135x86 bootloader version
136 137The complete bootloader version number. In the example above, this
138file will contain the value 564 = 0x234.
139 140See the type_of_loader and ext_loader_ver fields in
141Documentation/x86/boot.txt for additional information.
142 143==============================================================
144 145callhome:
146 147Controls the kernel's callhome behavior in case of a kernel panic.
148 149The s390 hardware allows an operating system to send a notification
150to a service organization (callhome) in case of an operating system panic.
151 152When the value in this file is 0 (which is the default behavior)
153nothing happens in case of a kernel panic. If this value is set to "1"
154the complete kernel oops message is send to the IBM customer service
155organization in case the mainframe the Linux operating system is running
156on has a service contract with IBM.
157 158==============================================================
159 160cap_last_cap
161 162Highest valid capability of the running kernel. Exports
163CAP_LAST_CAP from the kernel.
164 165==============================================================
166 167core_pattern:
168 169core_pattern is used to specify a core dumpfile pattern name.
170. max length 128 characters; default value is "core"
171. core_pattern is used as a pattern template for the output filename;
172 certain string patterns (beginning with '%') are substituted with
173 their actual values.
174. backward compatibility with core_uses_pid:
175 If core_pattern does not include "%p" (default does not)
176 and core_uses_pid is set, then .PID will be appended to
177 the filename.
178. corename format specifiers:
179 %<NUL> '%' is dropped
180 %% output one '%'
181 %p pid
182 %u uid
183 %g gid
184 %d dump mode, matches PR_SET_DUMPABLE and
185 /proc/sys/fs/suid_dumpable
186 %s signal number
187 %t UNIX time of dump
188 %h hostname
189 %e executable filename (may be shortened)
190 %E executable path
191 %<OTHER> both are dropped
192. If the first character of the pattern is a '|', the kernel will treat
193 the rest of the pattern as a command to run. The core dump will be
194 written to the standard input of that program instead of to a file.
195 196==============================================================
197 198core_pipe_limit:
199 200This sysctl is only applicable when core_pattern is configured to pipe
201core files to a user space helper (when the first character of
202core_pattern is a '|', see above). When collecting cores via a pipe
203to an application, it is occasionally useful for the collecting
204application to gather data about the crashing process from its
205/proc/pid directory. In order to do this safely, the kernel must wait
206for the collecting process to exit, so as not to remove the crashing
207processes proc files prematurely. This in turn creates the
208possibility that a misbehaving userspace collecting process can block
209the reaping of a crashed process simply by never exiting. This sysctl
210defends against that. It defines how many concurrent crashing
211processes may be piped to user space applications in parallel. If
212this value is exceeded, then those crashing processes above that value
213are noted via the kernel log and their cores are skipped. 0 is a
214special value, indicating that unlimited processes may be captured in
215parallel, but that no waiting will take place (i.e. the collecting
216process is not guaranteed access to /proc/<crashing pid>/). This
217value defaults to 0.
218 219==============================================================
220 221core_uses_pid:
222 223The default coredump filename is "core". By setting
224core_uses_pid to 1, the coredump filename becomes core.PID.
225If core_pattern does not include "%p" (default does not)
226and core_uses_pid is set, then .PID will be appended to
227the filename.
228 229==============================================================
230 231ctrl-alt-del:
232 233When the value in this file is 0, ctrl-alt-del is trapped and
234sent to the init(1) program to handle a graceful restart.
235When, however, the value is > 0, Linux's reaction to a Vulcan
236Nerve Pinch (tm) will be an immediate reboot, without even
237syncing its dirty buffers.
238 239Note: when a program (like dosemu) has the keyboard in 'raw'
240mode, the ctrl-alt-del is intercepted by the program before it
241ever reaches the kernel tty layer, and it's up to the program
242to decide what to do with it.
243 244==============================================================
245 246dmesg_restrict:
247 248This toggle indicates whether unprivileged users are prevented
249from using dmesg(8) to view messages from the kernel's log buffer.
250When dmesg_restrict is set to (0) there are no restrictions. When
251dmesg_restrict is set set to (1), users must have CAP_SYSLOG to use
252dmesg(8).
253 254The kernel config option CONFIG_SECURITY_DMESG_RESTRICT sets the
255default value of dmesg_restrict.
256 257==============================================================
258 259domainname & hostname:
260 261These files can be used to set the NIS/YP domainname and the
262hostname of your box in exactly the same way as the commands
263domainname and hostname, i.e.:
264# echo "darkstar" > /proc/sys/kernel/hostname
265# echo "mydomain" > /proc/sys/kernel/domainname
266has the same effect as
267# hostname "darkstar"
268# domainname "mydomain"
269 270Note, however, that the classic darkstar.frop.org has the
271hostname "darkstar" and DNS (Internet Domain Name Server)
272domainname "frop.org", not to be confused with the NIS (Network
273Information Service) or YP (Yellow Pages) domainname. These two
274domain names are in general different. For a detailed discussion
275see the hostname(1) man page.
276 277==============================================================
278 279hotplug:
280 281Path for the hotplug policy agent.
282Default value is "/sbin/hotplug".
283 284==============================================================
285 286kptr_restrict:
287 288This toggle indicates whether restrictions are placed on
289exposing kernel addresses via /proc and other interfaces. When
290kptr_restrict is set to (0), there are no restrictions. When
291kptr_restrict is set to (1), the default, kernel pointers
292printed using the %pK format specifier will be replaced with 0's
293unless the user has CAP_SYSLOG. When kptr_restrict is set to
294(2), kernel pointers printed using %pK will be replaced with 0's
295regardless of privileges.
296 297==============================================================
298 299kstack_depth_to_print: (X86 only)
300 301Controls the number of words to print when dumping the raw
302kernel stack.
303 304==============================================================
305 306l2cr: (PPC only)
307 308This flag controls the L2 cache of G3 processor boards. If
3090, the cache is disabled. Enabled if nonzero.
310 311==============================================================
312 313modules_disabled:
314 315A toggle value indicating if modules are allowed to be loaded
316in an otherwise modular kernel. This toggle defaults to off
317(0), but can be set true (1). Once true, modules can be
318neither loaded nor unloaded, and the toggle cannot be set back
319to false.
320 321==============================================================
322 323nmi_watchdog:
324 325Enables/Disables the NMI watchdog on x86 systems. When the value is
326non-zero the NMI watchdog is enabled and will continuously test all
327online cpus to determine whether or not they are still functioning
328properly. Currently, passing "nmi_watchdog=" parameter at boot time is
329required for this function to work.
330 331If LAPIC NMI watchdog method is in use (nmi_watchdog=2 kernel
332parameter), the NMI watchdog shares registers with oprofile. By
333disabling the NMI watchdog, oprofile may have more registers to
334utilize.
335 336==============================================================
337 338osrelease, ostype & version:
339 340# cat osrelease
3412.1.88
342# cat ostype
343Linux
344# cat version
345#5 Wed Feb 25 21:49:24 MET 1998
346 347The files osrelease and ostype should be clear enough. Version
348needs a little more clarification however. The '#5' means that
349this is the fifth kernel built from this source base and the
350date behind it indicates the time the kernel was built.
351The only way to tune these values is to rebuild the kernel :-)
352 353==============================================================
354 355overflowgid & overflowuid:
356 357if your architecture did not always support 32-bit UIDs (i.e. arm,
358i386, m68k, sh, and sparc32), a fixed UID and GID will be returned to
359applications that use the old 16-bit UID/GID system calls, if the
360actual UID or GID would exceed 65535.
361 362These sysctls allow you to change the value of the fixed UID and GID.
363The default is 65534.
364 365==============================================================
366 367panic:
368 369The value in this file represents the number of seconds the kernel
370waits before rebooting on a panic. When you use the software watchdog,
371the recommended setting is 60.
372 373==============================================================
374 375panic_on_unrecovered_nmi:
376 377The default Linux behaviour on an NMI of either memory or unknown is
378to continue operation. For many environments such as scientific
379computing it is preferable that the box is taken out and the error
380dealt with than an uncorrected parity/ECC error get propagated.
381 382A small number of systems do generate NMI's for bizarre random reasons
383such as power management so the default is off. That sysctl works like
384the existing panic controls already in that directory.
385 386==============================================================
387 388panic_on_oops:
389 390Controls the kernel's behaviour when an oops or BUG is encountered.
391 3920: try to continue operation
393 3941: panic immediately. If the `panic' sysctl is also non-zero then the
395 machine will be rebooted.
396 397==============================================================
398 399panic_on_stackoverflow:
400 401Controls the kernel's behavior when detecting the overflows of
402kernel, IRQ and exception stacks except a user stack.
403This file shows up if CONFIG_DEBUG_STACKOVERFLOW is enabled.
404 4050: try to continue operation.
406 4071: panic immediately.
408 409==============================================================
410 411 412pid_max:
413 414PID allocation wrap value. When the kernel's next PID value
415reaches this value, it wraps back to a minimum PID value.
416PIDs of value pid_max or larger are not allocated.
417 418==============================================================
419 420ns_last_pid:
421 422The last pid allocated in the current (the one task using this sysctl
423lives in) pid namespace. When selecting a pid for a next task on fork
424kernel tries to allocate a number starting from this one.
425 426==============================================================
427 428powersave-nap: (PPC only)
429 430If set, Linux-PPC will use the 'nap' mode of powersaving,
431otherwise the 'doze' mode will be used.
432 433==============================================================
434 435printk:
436 437The four values in printk denote: console_loglevel,
438default_message_loglevel, minimum_console_loglevel and
439default_console_loglevel respectively.
440 441These values influence printk() behavior when printing or
442logging error messages. See 'man 2 syslog' for more info on
443the different loglevels.
444 445- console_loglevel: messages with a higher priority than
446 this will be printed to the console
447- default_message_loglevel: messages without an explicit priority
448 will be printed with this priority
449- minimum_console_loglevel: minimum (highest) value to which
450 console_loglevel can be set
451- default_console_loglevel: default value for console_loglevel
452 453==============================================================
454 455printk_delay:
456 457Delay each printk message in printk_delay milliseconds
458 459Value from 0 - 10000 is allowed.
460 461==============================================================
462 463printk_ratelimit:
464 465Some warning messages are rate limited. printk_ratelimit specifies
466the minimum length of time between these messages (in jiffies), by
467default we allow one every 5 seconds.
468 469A value of 0 will disable rate limiting.
470 471==============================================================
472 473printk_ratelimit_burst:
474 475While long term we enforce one message per printk_ratelimit
476seconds, we do allow a burst of messages to pass through.
477printk_ratelimit_burst specifies the number of messages we can
478send before ratelimiting kicks in.
479 480==============================================================
481 482randomize_va_space:
483 484This option can be used to select the type of process address
485space randomization that is used in the system, for architectures
486that support this feature.
487 4880 - Turn the process address space randomization off. This is the
489 default for architectures that do not support this feature anyways,
490 and kernels that are booted with the "norandmaps" parameter.
491 4921 - Make the addresses of mmap base, stack and VDSO page randomized.
493 This, among other things, implies that shared libraries will be
494 loaded to random addresses. Also for PIE-linked binaries, the
495 location of code start is randomized. This is the default if the
496 CONFIG_COMPAT_BRK option is enabled.
497 4982 - Additionally enable heap randomization. This is the default if
499 CONFIG_COMPAT_BRK is disabled.
500 501 There are a few legacy applications out there (such as some ancient
502 versions of libc.so.5 from 1996) that assume that brk area starts
503 just after the end of the code+bss. These applications break when
504 start of the brk area is randomized. There are however no known
505 non-legacy applications that would be broken this way, so for most
506 systems it is safe to choose full randomization.
507 508 Systems with ancient and/or broken binaries should be configured
509 with CONFIG_COMPAT_BRK enabled, which excludes the heap from process
510 address space randomization.
511 512==============================================================
513 514reboot-cmd: (Sparc only)
515 516??? This seems to be a way to give an argument to the Sparc
517ROM/Flash boot loader. Maybe to tell it what to do after
518rebooting. ???
519 520==============================================================
521 522rtsig-max & rtsig-nr:
523 524The file rtsig-max can be used to tune the maximum number
525of POSIX realtime (queued) signals that can be outstanding
526in the system.
527 528rtsig-nr shows the number of RT signals currently queued.
529 530==============================================================
531 532sg-big-buff:
533 534This file shows the size of the generic SCSI (sg) buffer.
535You can't tune it just yet, but you could change it on
536compile time by editing include/scsi/sg.h and changing
537the value of SG_BIG_BUFF.
538 539There shouldn't be any reason to change this value. If
540you can come up with one, you probably know what you
541are doing anyway :)
542 543==============================================================
544 545shmmax:
546 547This value can be used to query and set the run time limit
548on the maximum shared memory segment size that can be created.
549Shared memory segments up to 1Gb are now supported in the
550kernel. This value defaults to SHMMAX.
551 552==============================================================
553 554shm_rmid_forced:
555 556Linux lets you set resource limits, including how much memory one
557process can consume, via setrlimit(2). Unfortunately, shared memory
558segments are allowed to exist without association with any process, and
559thus might not be counted against any resource limits. If enabled,
560shared memory segments are automatically destroyed when their attach
561count becomes zero after a detach or a process termination. It will
562also destroy segments that were created, but never attached to, on exit
563from the process. The only use left for IPC_RMID is to immediately
564destroy an unattached segment. Of course, this breaks the way things are
565defined, so some applications might stop working. Note that this
566feature will do you no good unless you also configure your resource
567limits (in particular, RLIMIT_AS and RLIMIT_NPROC). Most systems don't
568need this.
569 570Note that if you change this from 0 to 1, already created segments
571without users and with a dead originative process will be destroyed.
572 573==============================================================
574 575softlockup_thresh:
576 577This value can be used to lower the softlockup tolerance threshold. The
578default threshold is 60 seconds. If a cpu is locked up for 60 seconds,
579the kernel complains. Valid values are 1-60 seconds. Setting this
580tunable to zero will disable the softlockup detection altogether.
581 582==============================================================
583 584tainted:
585 586Non-zero if the kernel has been tainted. Numeric values, which
587can be ORed together:
588 589 1 - A module with a non-GPL license has been loaded, this
590 includes modules with no license.
591 Set by modutils >= 2.4.9 and module-init-tools.
592 2 - A module was force loaded by insmod -f.
593 Set by modutils >= 2.4.9 and module-init-tools.
594 4 - Unsafe SMP processors: SMP with CPUs not designed for SMP.
595 8 - A module was forcibly unloaded from the system by rmmod -f.
596 16 - A hardware machine check error occurred on the system.
597 32 - A bad page was discovered on the system.
598 64 - The user has asked that the system be marked "tainted". This
599 could be because they are running software that directly modifies
600 the hardware, or for other reasons.
601 128 - The system has died.
602 256 - The ACPI DSDT has been overridden with one supplied by the user
603 instead of using the one provided by the hardware.
604 512 - A kernel warning has occurred.
6051024 - A module from drivers/staging was loaded.
6062048 - The system is working around a severe firmware bug.
6074096 - An out-of-tree module has been loaded.
608 609==============================================================
610 611unknown_nmi_panic:
612 613The value in this file affects behavior of handling NMI. When the
614value is non-zero, unknown NMI is trapped and then panic occurs. At
615that time, kernel debugging information is displayed on console.
616 617NMI switch that most IA32 servers have fires unknown NMI up, for
618example. If a system hangs up, try pressing the NMI switch.
619